Obviously retracting the undercarriage has advantages to all aircraft but I was wondering why some helicopters have them..

For a fixed wing it seems to make more sense to me seeing as they usually travel further, have less moving parts than a helicopter and can help slow the aircraft down...but would having retractable gear make all that much of a difference to a helicopter? Or is it more of an appeal to have wheels that come up? The Augusta 109 looks very slick with wheels up

The Bell 430 comes in both a skid and retractable gear version; retractable gear gives about a 10 knot boost in speed at normal cruise power. Not trivial, but not enough to justify the added cost and weight if you are doing short hops. Many folks seem to think the retractable gear on the 430 was mainly an effort to appeal to the corporate market.

In some cases, operators who have helicopters with retractable gear prefer to keep them locked down for MX reasons as well as boosting safety.
For example, the IAF operates their CH-56's with wheels down all the time.

Also, I imagine although there is a big gain from having a RG helicopter in terms of drag reduction - you have to consider the fact the RG system itself is a lot heavier than having just skids.

So I guess there's some balancing there somewhere to be done, there must be a break even point where the advantage of the drag reduction outweighs the advantage of having very light skids instead of a gear retraction mechanism.